Instructions
Flip the envelope to its front side. Look for a solid sheet of paper -- the rear side has an opening flap for slipping contents inside the envelope.
Locate the upper right-hand corner of the envelope's front side. The flap on the rear of the envelope opens at the top -- orient the envelope and find the upper right corner using that as a reference point.
Affix the appropriate number of stamps to the upper right-hand corner. The number of stamps needed depends on the weight of the envelope or package -- use the U.S. Postal Service's online rate calculator or its price list to determine how many stamps are needed. Place multiple stamps next to each another and avoid overlapping or covering one stamp with another.